Use of Advanced Borehole Geophysical Techniques to Delineate
Fractured-Rock Ground-Water Flow and Fractures Along Water-Tunnel Facilities
in Northern Queens County, New York

U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 00-4276

ABSTRACT

Advanced borehole geophysical
methods were used to assess the geohydrology of
crystalline bedrock along the course of
a new water tunnel for New York City. The
logging methods include natural gamma, spontaneous
potential, single-point resistance, mechanical and
acoustic caliper, focused electromagnetic induction,
electromagnetic resistivity, magnetic susceptibility,
borehole-fluid temperature and conductance, differential
temperature, heat-pulse flowmeter, acoustic televiewer,
borehole deviation, optical televiewer, and borehole
radar. Integrated interpretation of the geophysical logs
from an 825-foot borehole (1) provided information
on the extent, orientation, and structure (foliation and
fractures) within the entire borehole, including intensely
fractured intervals from which core recovery may be poor;
(2) delineated transmissive fracture zones intersected by
the borehole and provided estimates of their transmissivity
and hydraulic head; and (3) enabled mapping of the
location and orientation of structures at distances as much
as 100 ft from the borehole.

Analyses of the borehole-wall image and the
geophysical logs from the borehole on Crescent Street, in
northern Queens County, are presented here to illustrate
the application of the methods. The borehole penetrates
gneiss and other crystalline bedrock that has predominantly
southeastward dipping foliation and nearly horizontal and
southeastward-dipping fractures. The heat-pulse flowmeter
logs obtained under pumping and nonpumping conditions,
together with the other geophysical logs, indicate five
transmissive fracture zones. More than 90 percent of the
open-hole transmissivity is associated with a fracture zone
272 feet BLS (below land surface). A transmissive zone
at 787 feet BLS that consists of nearly parallel fractures
lies within the projected tunnel path; here the hydraulic
head is 12 to 15 feet lower than that of transmissive zones
above the 315-foot depth. The 60-megahertz directional
borehole radar logs indicate the location and orientation of
two closely spaced radar reflectors that would intersect the
projection of the borehole below its drilled depth.

Subsequent excavation of the tunnel past the borehole
allowed comparison of the log analysis with conditions
observed in the tunnel. The tunnel was found to intersect
gneiss with southeastward dipping foliation; many nearly
horizontal fractures; and a southeastward dipping fracture
zone whose location, character, and orientation was
consistent with that of the mapped radar reflectors. The
fracture zone produced inflow to the tunnel at a rate of
50 to 100 gallons per minute. All conditions indicated by
the logging methods were consistent with those observed
within the tunnel.